Isomerization processes are widely used by many refiners to rearrange the molecular structure of straight chain paraffinic hydrocarbons to more highly branched hydrocarbons that generally have higher octane ratings. One such process rearranges normal butane (n-butane) to isobutane in a paraffin isomerization-zone. The isomerization process proceeds toward a thermodynamic equilibrium in which the effluent (isomerate) still contains a substantial concentration of n-butane, typically in the range of a mole ratio of isobutane to n-butane of from about 1.2:1 to about 2:1. Downstream from the paraffin isomerization-zone, the effluent is separated in a deisobutanizer (DIB) column to form an isobutane product, usually having a purity of at least about 80 mole % and up to about 99 mole % isobutane. The DIB column is configured as a fractional distillation column with a plurality of vertically aligned and spaced apart fractionation trays that effectively act as vapor-liquid contacting devices for fractionating the effluent.
As the boiling points of n-butane and isobutane are relatively close and a relatively pure isobutane product is desired, DIB columns typically operate with a high reflux ratio to help drive the vapor fractions of the effluent up through the DIB column contacting the descending liquid fractions of the effluent to effectively separate the isobutane product. Unfortunately, in efforts to be more energy efficient, some DIB columns are designed with significantly lower reboiling duties in the lower portion of the column compared to the upper portion of the column, e.g., ratio of the reboiling duties of the lower portion relative to the upper portion of the column of from about 1:9 to about 1:10. As such, significantly less vapor is generated in the lower portion of the DIB column than in the upper portion of the column. This can result in lower vapor velocities and flow of liquid through the holes on the fractionation trays called weeping, leading to inefficient separation of the paraffin isomerization-zone effluent and a lower purity isobutane product.
Accordingly, it is desirable to provide apparatuses and methods for separating paraffin isomerization-zone effluents with improved vapor velocity and reduced weeping in DIB columns for efficient separation of isobutane from the effluent. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.